Nickel iron battery employing an untreated polyolefin separator with a surfactant in the electrolyte

ABSTRACT

Provided is a nickel-iron battery. The battery comprises a positive nickel electrode, an iron negative electrode, an electrolyte comprising a surfactant, and a non-polar separator. In one embodiment, the non-polar separator is comprised of a polyolefin, and the surfactant comprises a zwitterionic surfactant.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 61/761,312, filed Feb. 6, 2013; and U.S. Provisional ApplicationSer. No. 61/907,958, filed Nov. 22, 2013, which applications areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is in the technical field of energy storagedevices. More particularly, the present invention is in the technicalfield of rechargeable batteries employing an iron electrode, and theseparators used in those batteries.

2. State of the Art

The nickel iron (Ni—Fe) battery was independently developed by Edison inthe United States and by Junger in Sweden in 1901. It was industriallyimportant from its introduction until the 1970's when batteries withsuperior specific energy and energy density replaced Ni—Fe batteries inmany applications.

However, Ni—Fe batteries have many advantages over other batterychemistries. The Ni—Fe battery is a very robust battery which is verytolerant of abuse such as overcharge and overdischarge and can have avery long life. It is often used in backup situations where it can becontinuously trickle-charged and last more than 20 years. Additionally,the active material iron is much less expensive than active materialsused in other alkaline battery systems such as NiMH or in non-aqueousbatteries such as Li Ion. However, the low specific energy, low energydensity, and poor power have limited the applications of this batterysystem.

The Ni—Fe battery is a rechargeable battery having a nickel(III)oxy-hydroxide positive electrode and an iron negative electrode, with analkaline electrolyte such as potassium hydroxide. The overall cellreaction can be written as:

2 NiOOH+Fe+2 H₂O

2 Ni(OH)₂+Fe(OH)₂   (1)

The ability of these batteries to survive frequent cycling is due to thelow solubility of the reactants in the electrolyte. The formation ofmetallic iron during charge is slow due to the low solubility of thereaction product ferrous hydroxide. While the slow formation of ironcrystals preserves the electrodes, it also limits the high rateperformance. Ni—Fe cells are typically charged galvanostatically andshould not be charged from a constant voltage supply since they can bedamaged by thermal runaway. Thermal runaway occurs due to a drop in cellvoltage as gassing begins due to overcharge, raising the celltemperature, increasing current draw from a constant potential source,further increasing the gassing rate and temperature.

As shown in Equation (1), the overall cell reaction does not involve thebattery electrolyte; however, alkaline conditions are required for theindividual electrode reactions. Therefore, iron-based batteries such asNi—Fe, Fe-air, and Fe—MnO₂ batteries all employ a strong alkalineelectrolyte typically of KOH, typically in the range of 30-32% KOH. KOHis typically employed due to its higher conductivity and low freezingpoint. LiOH may be added in cells subject to high temperatures due toits stabilization effects on the nickel electrode, improving its chargeacceptance at elevated temperatures.

The separators used in the cell construction depends upon the types ofelectrodes used. In Ni—Fe cells with a pocket plate electrodes, theanode and cathode are kept electrically isolated using a spacer or agrid-like mesh inlay and are typically held in a rigid frame. However,the construction of these cells is more expensive as the electrodedesign is not amenable to lower-cost manufacturing methods. Furthermore,the large inter-electrode spacing of these batteries imposed by therigid support limits high rate performance.

Cells constructed with plastic-bonded, sintered, fiber, or foamelectrodes are often lower in cost than cells with pocket plateelectrodes. The electrode manufacturing process is cheaper, easier, andprovides greater consistency between electrodes than the pocket platedesign. Unlike Ni—Fe cells with a pocket plate electrodes, these cellsuse microporous separators. These cells have other advantages besideslower cost such as higher rate capability and greater energy densitysince microporous separators are much thinner which keeps theinter-electrode spacing small as the electrodes are held in placethrough compression. Microporous separators that consist ofpolypropylene, polyethylene, and polyolefin blends have been used inalkaline battery systems. The non-polar nature of the polyolefin chainmakes it difficult to wet the separator. This problem has been addressedby modifying the surface properties of the polyolefin materials used toform polymeric sheets, by graft-copolymerizing to those surfaces amonomeric substance which, after copolymerization, confers hydrophilicproperties and, in some cases, ion exchange properties.

However, it has been observed that a failure mechanism for Ni—Fe cellsappears to be iron or iron oxide accumulation on the separator. Theaccumulation can lead to an electrical short between the anode andcathode. Such an accumulation of iron or iron oxide leads to a prematurefailure of the cell.

An object of this invention is to improve the cell life of a batterycomprising an iron electrode. Another object is to provide aseparator/electrolyte combination that overcomes the problems ofpremature failure of a Ni—Fe cell. Providing such a solution would be ofgreat value to the industry.

SUMMARY OF THE INVENTION

Provided is a nickel-iron battery comprising a nickel positiveelectrode, an iron negative electrode, an electrolyte comprising asurfactant, and a non-polar separator. By utilizing a separator is thatcomprised of a non-polar material on its surface along with asurfactant, the cell life of a Ni—Fe cell is increased. The wettabilityof the nonpolar separator by an alkaline electrolyte is also improved byusing an electrolyte comprising a surfactant.

Among other factors, it has been found that by using an alkalineelectrolyte comprising a surfactant in a Ni—Fe cell when a non-polarseparator is used, the combination of the electrolyte with thesurfactant and non-polar separator results in an increased cell life.Generally, the non-polar separator can be treated for wettability.However, the non-polar separator can be totally untreated in the presentinvention yet achieve a Ni—Fe cell with increased cell life.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING

FIG. 1 is an image of magnetite deposits that were found coating thefibers of the separator of a Ni—Fe cell with polyolefin separatorsconsisting of polypropylene whose surface was treated to improve itswettability.

FIG. 2 shows the capacity versus cycle number during an accelerated lifetest for nickel-iron cells that have a polypropylene separator treatedwith acrylic acid grafting to improve wetting (solid line), an untreatedpolypropylene separator (dashed line), and an untreated polypropyleneseparator in a cell with an electrolyte containing 1% mirataine.

DETAILED DESCRIPTION OF THE INVENTION

The invention comprises a Ni—Fe cell with a separator that is non-polar,such as a polyolefin, and an alkaline electrolyte that comprises asurfactant. During testing of different separators and electrolytesolutions it was unexpectedly observed that separators with no surfacetreatment in combination with an electrolyte containing a surfactant hadimproved life compared to cells with a separator whose surface wastreated for improved wettability and cells that had a polyolefinseparator whose surface was not treated to improve wettability but hadno surfactant. The battery may be prepared by conventional processingand construction employing a nickel oxyhydroxide positive electrode, analkaline electrolyte, and iron electrode. The nickel electrode may be ofa sintered type well known in the art or may be of a pasted typeemploying a foam or felt matrix. The battery electrolyte may comprise aKOH solution, or alternatively is a NaOH based electrolyte, and alsocomprises a surfactant. The separator is a polyolefin that has not beentreated to increase wettability. The iron electrode may be of a pasteddesign comprising a single layer of substrate coated with a binder andan iron active material comprising iron metal or iron oxides.

Microporous separators that are comprised of polypropylene,polyethylene, and polyolefin blends are commonly used in alkalinebattery systems. The non-polar nature of the polyolefin chain makes itdifficult to wet the separator. The wettability is increased bygraft-copolymerizing to those surfaces a monomeric substance which,after copolymerization, confers hydrophilic properties. The graftedspecies often contains a carboxylic acid group, carboxylate, orsulfonate that when placed in an alkaline electrolyte environment ismostly the cation of the electrolyte charge carrier as its cation. Thus,in a NaOH based alkaline electrolyte, the counter-ion for a carboxylateor sulfonate group on the separator after immersion in the electrolytewould primarily be Na⁺. However, by modifying the surfaces in such amanner, the surface of the separator may take on the ability to allowion-exchange which could allow iron ions to accumulate at the separator.

Ni—Fe cells that had reached their end-of-life were autopsied opened andthe components were inspected. Magnetite, Fe₃O₄ deposits were foundcoating the fibers of the separator of a Ni—Fe cell with polyolefinseparators consisting of polypropylene whose surface was treated toimprove its wettability. Magnetite may have been the original deposit ormay have originated from iron deposits that oxidized upon exposure toair prior to analysis, see FIG. 1. During autopsy it was also observedthat the separator was adhered to the electrodes. Furthermore, it wasfound that Ni—Fe cells could be rejuvenated by draining the electrolyte,replacing the separator, and then refilling the cell with the drainedelectrolyte. This suggests that the deposition of iron or iron compoundson the separator is responsible for cell failure. It is believed thatfunctionalization of the separator by grafting hydrophilic groups toincrease its wettability also allows for ion-exchange where iron ionscould collect and lead to eventual iron accumulation.

While not wishing to be bound by any theory, it is believed that duringdischarge, iron is oxidized forming iron cations that have a slightsolubility in the alkaline electrolyte. These iron cations may undergoion-exchange with the cations associated with functional groups on aseparator whose surface has been modified to improve wettability. Thedeposited iron cations may be either unstable in the alkalineelectrolyte and form magnetite or might be reduced to iron metal byhydrogen gas that is generated during charging of the cell causing abuild-up of iron on the separator. Cells with a separator having nosurface treatment would suffer from wetting issues.

In the present invention, the non-wettability of untreated polyolefinshas been overcome by treating the polyolefin material with a surfactant.The surfactant is contained in the alkaline electrolyte, which allows anaqueous electrolyte to wet the separator. However, such surfactant canbe removed or lost from the surface of the polyolefin separator materialwhen electrolyte is lost from the device. For example, this can happenduring charging and discharging cycles, if the surfactant is notsubsequently replaced on the material when the electrolyte isreplenished.

The nickel-iron battery of the present invention comprises an untreatednon-polar, e.g., polyolefin separator and an electrolyte that contains asurfactant. Suitable polyolefins include, but are not limited to:polyethylene (including, for example, LDPE, LLDPE, HDPE, UHDPE),polypropylene, polybutylene, polymethylpentane, co-polymers thereof, andblends thereof. The separators may comprise layered sheets of materials.The separator has a nonpolar surface and is not chemically treated toimprove wettability to aqueous electrolytes.

The alkaline electrolyte of the battery comprises a surfactant toimprove the wettability of the untreated polyolefin separator. Lowfoaming surfactants are preferred. The surfactant may be an anionicsurfactant possessing anionic functional end groups, such as a sulfate,sulfonate, phosphate, and carboxylates. Cationic surfactants may also beused where a tertiary amine is an end group. Zwitterionic surfactantssuch as sultaines having both cationic and anionic centers attached tothe same molecule may also be used. In a preferred embodiment, RhodiaMirataine® ASC Surfactant consisting of an alkylether hydroxypropylsultaine is used.

The following examples are provided to further illustrate the presentinvention. The examples are meant to be illustrative, and not limiting.

EXAMPLES Example 1

The positive electrodes of nickel iron batteries were constructed fromsintered nickel electrodes impregnated with nickel hydroxide. Thenegative electrode in the batteries was a pasted design comprising asingle layer of substrate coated with a binder and an iron activematerial comprising iron metal. The electrolyte used comprised 30% KOHwith 1 N LiOH. The separator was 4.5 mil thick microporous polypropylenetreated with acrylic acid grafting, which results in oligomeric acrylicacid on the polymer chain. After initial formation and performancetesting, cells were put on an accelerated life test. Two cells weretested. The results are shown in FIG. 2.

Example 2

The positive electrodes of nickel iron batteries were constructed fromsintered nickel electrodes impregnated with nickel hydroxide. Thenegative electrode in the batteries was a pasted design comprising asingle layer of substrate coated with a binder and an iron activematerial comprising iron metal. The electrolyte used comprised 30% KOHwith 1 N LiOH. The separator was 4.5 mil thick microporous polypropylenethat was not treated to improve wetting. After initial formation andperformance testing, cells were put on an accelerated life test. Twocells were tested. The results are shown in FIG. 2.

Example 3

The positive electrodes of nickel iron batteries were constructed fromsintered nickel electrodes impregnated with nickel hydroxide. Thenegative electrode in the batteries was a pasted design comprising asingle layer of substrate coated with a binder and an iron activematerial comprising iron metal. The electrolyte used comprised 30% KOHwith 1 N LiOH with 1% Rhodia MIRATAINE® ASC Surfactant. The separatorwas 4.5 mil thick microporous polypropylene that was not treated toimprove wetting. After initial formation and performance testing, cellswere put on an accelerated life test. Two cells were tested. The resultsare shown in FIG. 2.

From the results shown in FIG. 2, it can be seen that the nickel-ironcell of Example 3, using an untreated polypropylene separator and anelectrolyte with a surfactant, provided increased cell life.

While the foregoing written description of the invention enables on ofordinary skill to make and use what is considered presently to be thebest mode thereof, those of ordinary sill will understand and appreciatethe existence of various, combination, and equivalents of the specificembodiment, method and examples therein. The invention should thereforenot be limited by the above described embodiment, method and examples,but by all embodiments and methods within the scope and spirit of theinventions and the claims appended therein.

What is claimed is:
 1. A nickel-iron battery comprising a nickelpositive electrode, an iron negative electrode, an electrolytecomprising a surfactant, and a non-polar separator.
 2. The nickel-ironbattery of claim 1, wherein the separator is comprised of a polyolefin.3. The nickel-iron battery of claim 2, wherein the separator iscomprised of polypropylene or polyethylene.
 4. The nickel-iron batteryof claim 1, wherein the electrolyte comprises NaOH.
 5. The nickel-ironbattery of claim 1, wherein the surfactant is a low-foaming electrolyte.6. The nickel-iron battery of claim 1, wherein the surfactant comprisesan anionic surfactant.
 7. The nickel-iron battery of claim 1, whereinthe surfactant comprises a cationic surfactant comprising a tertiaryamine.
 8. The nickel-iron battery of claim 1, wherein the surfactant isa zwitterionic surfactant.
 9. The nickel-iron battery of claim 8,wherein the surfactant comprises a sultaine.
 10. The nickel-iron batteryof claim 9, wherein the surfactant comprises an alkylether hydroxypropylsultaine.